Embodiments in accordance with the present invention relate to a magnetic head mounted in a magnetic storage system, and in particular relate to a magneto-resistive head which reads information recorded on a magnetic recording medium, and to a method of manufacturing the same.
At present, as a magneto-resistive head mounted as a read sensor in a magnetic storage system, a GMR (giant magneto-resistive) head using a spin-valve stack having a basic structure comprising a magnetic layer/nonmagnetic conductive layer/magnetic layer/antiferromagnetic layer as disclosed in JP 4(1992)-358310 A, is widely employed. In the spin-valve stack, the magnetic layer whereof the direction of magnetization is fixed unidirectionally by exchange coupling with the antiferromagnetic layer is referred to as a pinned magnetic layer, and the other magnetic layer whereof the magnetization direction can be varied freely according to an external magnetic field is referred to as a free magnetic layer.
A GMR head having a spin-valve stack uses a phenomenon whereby the electrical resistance varies according to the angle between the magnetizations of the pinned magnetic layer and the free magnetic layer, and outputs a magnetic signal as a voltage variation or current variation. Therefore, an important function of the spin-valve stack, in addition to functioning as a magnetic sensor, is to fix the direction of magnetization of the pinning layer in one direction (specifically, the direction perpendicular to the magnetic recording medium, referred to hereafter as “sensor height direction”).
On the other hand, to obtain symmetrical response characteristics relative to the sign of the signal magnetic field, the magnetization of the free magnetic layer must be oriented in the track width direction when the external magnetic field is zero. Further, to obtain a good linear response with little noise, a longitudinal biasing field must be applied in the track width direction relative to the free magnetic layer so that the free magnetic layer has a single domain structure. Regarding this longitudinal biasing field, JP 7(1995)-57223 A discloses that a hard magnetic film or a film stack of a magnetic film and antiferromagnetic film is disposed at both ends of the spin-valve stack, and a longitudinal biasing field is applied to a free magnetic layer to form a single domain structure. In particular, the former is referred to as a hard bias structure, and presently accounts for most of the GMR head structures in use.
In the aforesaid GMR head, a sensor current is passed in the in-plane direction of the spin-valve stack, and the head is therefore referred to as a CIP (Current In the Plane) GMR head. A head which has still higher reading sensitivity than the CIP-GMR head is a CPP (Current Perpendicular to the Plane) GMR head or a TMR (Tunneling Magneto-Resistive) head. In both of these cases, the basic mode of operation is identical, it being required to obtain stable reading characteristics by applying a longitudinal biasing field based on a hard bias structure.
Further, U.S. Pat. No. 5,408,377 discloses a “synthetic ferrimagnetic free layer”. Here, the free magnetic layer comprises a magnetic layer/interlayer antiferromagnetic coupling layer/magnetic layer, and the effective magnetization amount can be reduced by arranging the two magnetic layers to have an antiparallel alignment. In this disclosure, the two magnetic layers are given a very strong antiferromagnetic coupling, and intentionally given different magnetizations. Hence, when the signal magnetic field from the magnetic recording medium is detected, the magnetization direction can be varied while the two magnetic layers maintain an antiparallel alignment.
The longitudinal biasing field converts the free magnetic layer to a single domain structure and is effective in suppressing noise. The anisotropy field of the free magnetic layer is generally no more than 1 kA/m, and since it becomes magnetically saturated very easily, in order to obtain a linear response relative to the signal magnetic field from the magnetic recording medium, the effective anisotropy field of the free magnetic layer magnetization must be increased by applying the longitudinal biasing field. This therefore means that reading characteristic sensitivity and stability are in a trade-off relationship depending on the magnitude of the longitudinal biasing field. In other words, if the longitudinal biasing field is too strong, reading output falls, while on the other hand if the longitudinal biasing field is too weak, a sufficient single domain magnetization effect is not obtained and noise increases. Hence, in order to obtain satisfactory reading characteristics, the magnitude of the longitudinal biasing field must be optimized. In a hard bias structure, the magnitude of the longitudinal biasing field is determined by plural factors which interact in a complex way such as (1) the ratio of the magnetization amounts of the free magnetic layer and the hard magnetic film, (2) the etched shape of the spin-valve stack at the end of the track, and (3) the physical spatial relationship of the hard magnetic film relative to the free magnetic film, so it is very difficult to suppress these factors. In addition, to position the hard magnetic film, since the shield-shield spacing at the track ends increases relative to the center part, the problem of “side reading” occurs, the effective magnetic track width does not become narrower even if the physical track width is narrowed, and future high magnetic recording densities will not be achieved. This problem is particularly marked, the narrower the track becomes.
In the future, with increased surface recording density of magnetic storage systems, it is expected that as the track width becomes narrower, due to the aforesaid reason, in reading heads with a hard bias structure, it will be difficult to simultaneously achieve a high reading sensitivity and satisfactory linear response characteristics with low noise. In particular, in CPP reading heads where high reading sensitivity is expected, since an insulation film must the interposed between the spin-valve stack and the hard magnetic film, a sufficiently large longitudinal biasing field cannot be applied from the hard magnetic film and stability may be impaired.